Sound reproducing system



July 23, 1935. J. H; HAMMOND, JR 2,003,707

SOUND REPRODUCING SYSTEM Original Filed Jan. 22, 1951 2 Sheets-Sheet 1 A TTORNEYS v July 23, 1935. J. H. HAMMOND, JR

SOUND REPRODUCING SYSTEM Original Filed Jan. 22, 1931 2 Sheets-Sheet 2 5 :Ezi

o fluQ m ago 0 From Transformer 2! lnpuf Decibels %7% 7LINVNTOR WM BY 2 ATTORNEYS Patented July 23, 1935 2,008,707 SOUND REPRODUCING SYSTEM John Hays Hammond, Jr., Gloucester, Mass.

Application January 22, 1931, Serial No. 510,386 Renewed September 22, 1933 12 Claims.

This invention relates to sound reproducing systems, and more particularly to an improved system for automatically expanding or multiplying the dynamic range of the sound energy.

The invention provides more specifically for the simultaneous control of the gain ratio or amplification of a plurality of cascaded stages of amplification in accordance with the energy applied to the system whereby the volume ratio of the signal may be expanded.

The invention further provides a limiting control for the system which may be so arranged that the amplifier will operate for strong signals under condition for maximum output without tonal distortion. For this purpose various controls are arranged so that the gain ratio may be varied as any predetermined function of the strength of the input signal. For example, the gain ratio may be made to increase rapidly up to a certain point after which the gain ratio may be caused to increase less rapidly or to remain constant. In the reproduction of speech, for example, it may be preferable to cause the gain ratio to increase rapidly when the speech level exceeds the level of the ground noise and to cause the device to remain practically inoperative when only the signals of ground noise intensity are present. Certain musical selections on the. other hand require a wide dynamic range and in reproducing the same it is frequently desirable to cause the gain ratio of the amplifiers to increase fairly constantly as a function of the input signal strength up to the point where distortion would occur due to the inherent characteristics of the amplifier. The present invention provides for making the necessary adjustments to effect these various controls whereby the desired amplification characteristics may be obtained.

The invention also consists in certain new and original features of construction and combinations of parts hereinafter set forth and claimed.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof, in which:

Figure 1 illustrates diagrammatically the system as applied to the reproduction of the sound record of a talking moving picture projector and 55 phonograph; and

Figure 2 shows typical curves illustrating the operation of the system.

In the following description and in the claims,

parts will be identified by specific names for convenience, but they are intended to be as generic in their application to similar parts as the art will permit.

Referring to the accompanying drawings, a phonograph pick up i0 is shown which is connected to two contacts of a double-pole doublethrow switch H, the other contacts of which are connected through a, transformer 12 to a photoelectric cell 53. Light from a lamp I4 is focused, by means of a lens 85, through an aperture 86 in an aperture plate ill, upon the sound record of a moving photographic film l8. The light which passes through this film then impinges on the photoelectric cell 93.

The blades of the switch it are connected through a transformer 28 to a potentiometer 22,

which is connected in the input circuit of a space discharge amplifier 24 including tube 24. The output circuit of this amplifier includes the primary of a transformer 25 which is shunted by a resistance 26. The secondary of the transformer 25 is connected in the input circuit of a push pull amplifier 21, including tubes E27, the output circuit of which includes the primary of a transformer 28, which is shunted by a resistance 29. The resistances 26 and 29 are designed to maintain the frequency characteristics of the circuits substantially independent of the gain ratio characteristics. A meter 30 may be inserted in theplate circuit of the push pull amplifier 2? to facilitate adjustment of the system. The secondary of the transformer 28 is connected to a power amplifier 3!, the output of which is connected to a plurality of loud speakers 32, 33.

It is to be understood that additional amplifiers may be inserted in the photoelectric pick up circuit, as for example between transformer 82 and switch it, if desired.

The potentiometer 22 is shunted by a bank of condensers 23 for controlling the tone quality. The secondary of the transformer 26 'is connected through a stopping condenser 36 to a potentiometer which is connected in the input circuit of space discharge device 39, the output circuit of which is connected -to the primary of a transformer ll. The secondary of transformer ti is connected to a circuit including a rectifier 8t, resistance and a tapped resistance Q6.

The cathodes of tubes l2? are joined in parallel and connected to ground, that is, the junction of the plate supply battery and the grid bias battery. The cathode of tube I24 is joined to a point somewhat positive with respect to ground as determined by a tap on the'plate supply resistor assembly. The grid return circuit of device I24 is connected to the junction point of resistors 45 and 46, whereas the grid return circuit of device I21 is taken from the tap onresistor 46. Thus, in the absence of direct current through resistor 46, the grids are at equal potentials in all three devices, but the bias on tube I24 is slightly in excess of that on tubes I21 because of the difference of cathode connections.

Suitable by-pass condensers 49, 41 and 48 are provided across resistance 46 forming with resistance 45 a filter to eliminate tone currents from the control circuit. A double-pole doublethrow switch 52 is used to provide a low initial bias for the amplifiers from an adjustable contact 53 on a resistor assembly 54 when it is not desired to use the dynamic multiplier. In the other position of the switch 52 a bank of condensers 55 is connected across the resistance 46.

A source of potential is provided, which is supplied through a transformer 56 from a source of A. C. 51. The filaments of the devices I24 and 39, and I21, are supplied with current from a secondary of the transformer 56. Another secondary of this transformer supplies current to the filament of the rectifier 44. An adjustable contact 58 on a resistance 59 is used for providing the initial biasing voltage on the devices I24 and I21.

It will be noted that the plate applied voltage for tubes I21 is slightly less than that for tube I 24 because of the difference of cathode connections. However, this is a negligible factor in determining the relative plate currents of the three devices.

In this invention a limiter is provided which consists of a space discharge device 6I, the cathode of which is connected to the adjustable contact 53 of resistance 54, and the grid and plate of which are connected to the positive end of resistor 46, through a switch 62.

In the operation of the form of the invention shown, with no applied signal, the voltage on the grids of the amplifier I24 and the push pull amplifier I21 is determined by the setting of the adjustable contact 58 on resistance 59, which is so positioned that small plate current will flow through the push pull amplifier I21, as indicated by the reading of the meter 30. The plate current through the amplifier I24 will be greater than through the push pull amplifier I21, due to the fact that the cathode of the device I24 is at a higher voltage than the cathodes of the push pull amplifier I21 until sufficient current passes through the resistor 46 to cause a reversal of conditions.

When a signal is received from either the phonograph pick up II] or the photoelectric cell I3, energy will pass through the transformer 2I to the potentiometer 22 by which any desired amount of energy is fed to the input circuit of the amplifier 24, where it will be amplified and passed through the transformer 25 to the input circuit of the push pull amplifier 21. Part of the energy passing through the transformer 2| passes through the potentiometer 35, to the input circuit of the control device 39. The output of this device passes through the transformer 4| and is rectified by the rectifier 44. This rectified current passes through the resistance 46 in the direction of the arrows. thus causing a potential difference to be built up across this resistance which is an increasing function of the input-signal strength. This potential difference will oppose the biasing voltages of the amplifiers 24 and 21, as determined by the setting of the contact 58, thus decreasing the bias on these devices, which increases the amplification factors of these stages.

This action is limited by the device 6| which is not brought into operation until the potential at the positive end of the resistance 46 has become greater than the potential of the cathode of the device 6 I, as determined by the setting of the contact 53. When this occurs, the device 6| will allow current to pass in the direction of the arrow,

thus acting as a variable resistance in parallel with the resistance 46. It has been found that with rectifier 44 of low impedance compared with its load resistance, the rectified power is quite independent of the load resistance value over a considerable range. Thus if the resistance load is of high impedance, a high voltage is produced. When device 6| passes current, the voltage across 46 builds up less slowly or not at all when further signal is applied, due to the lowering of the load resistance by the shunting action of device 6|. In this way the potential built up across the resistance 46 is limited and, therefore, with suitable setting of potentiometer 53, underbiasing of the device 21 is prevented.

As the change of bias on the amplifier 24 is determined by the full potential drop across the resistance 46, it will be greater than the change of bias on the push pull amplifier 21 which is determined by the potential drop across only a part higher than that on the amplifier 21, the bias.

voltage on the amplifier 24 may vary from 20 volts to 8 volts, a range of 12 volts, while that on the amplifier 21 may vary from 18 volts to 10 volts, a range of 8 volts. It is thus seen that the *bias on the amplifier 24 will start at a higher value and end at a lower value than the bias on the amplifier 21 and may have a greater range. The reason for this difference in bias range is that the second amplifier receives a greater input signal strength than the first and, therefore, its amplification range can not-be so great without producing distortion.

The output of the push pull amplifier 21 passes through the transformer 28 to the input circuit of the high power amplifier 3|, there being amplified and fed to the loud speakers 32 and 33, where it is reproduced as speech or music in the usual manner.

As a result the dynamic range of the amplifier stage including device I 24 is greater than. the dynamic range of the amplifier stage including device I21. For example, in arranging for a total expansion of 24 decibels an overall change of gain ratio from 10 to 1 is required. The present invention distributes the change of gain ratio between the stages in accordance with their signal carrying capacity, as for example, 5.33 to 1 in the first stage and 3 to 1 in the second stage.

The potentiometer 22 controls the input signal strength and, therefore, the total possible volume of the system, but has no control over the dynamic multiplication which is controlled independently by the potentiometer 35. It is thus seen that in this system it is possible to control the volume and dynamic multiplication independently. In particular, potentiometer 35 in combination with the initial bias adjustment 58 provides that even though the tonal output of the sound record is high above average, the minimum bias for tubes I21 may be that for which they can deliver the maximum undistorted power output.

When the system is to be used chiefly for speech, the switch 52 is left open, and when used for music, this switch is thrown down, thus throwing the condensers 55 across the resistance 46. By selecting the proper condenser in the bank 55, it is possible to vary the rate of operation of the dynamic multiplier. For example, by increasing the capacity across the resistance 46, the action of the dynamic multiplier is retarded.

If it is desired to eliminate the dynamic multiplier entirely, the switch 52 is thrown up, thus applying a small negative voltage from the ad justable contact 54 to the grids of the amplifiers 24 and 21 which then operate as normal amplifiers.

Figure 2 depicts various relations between input and output decibels that may be achieved by this invention, with a fixed setting of potentiometer 22 which determines the input signal to the amplifier. Line ll depicts the condition when the circuit is set for normal amplification with switch 52 up, and contact 53 adjusted to give a fixed bias for tubes I26 and I2? suitable for maximum undistorted output. The minimum usable signal input from transformer 2! is 68 decibels, and maximum 32, whereby the usable signal is of 36 decibels range. Because of the linear amplification, the output decibel range for curve ll is the same as the input decibel range, giving on strongest signal +12 decibels output.

Without change of potentiometer 22, and with switch 62 open, the switch 52 either open or down to cut in additional timing condensers, potentiometer 58. may be so adjusted as to give low amplification for weak signals. For example at an input of 68 decibels, the amplifier may produce only 48 decibels output, whereas the output for 68 decibels input on curve H was 24'decibels. That is contact 58 is adjusted to give 24 additional decibels of attenuation for 68 decibels input. Potentiometer 35 may now be set to give +12 decibels output on the strongest signals of the record. This strongest signal causes the minimum bias on tubes H25 and i2? to be roughly the same as in curve ll. Curve l2 depicts the relation between input and output decibels, for signals intermediate between the lowest usable signal of the record, and the highest signal.

For bringing limiter Bi into operation. switch 62 is closed. By suitable adjustment of dynamic control potentiometer 35 which determines the rate of bias change with change of voltage across secondary of transformer 2i, and by suitable adjustment of limiter threshold potentiometer 53 which determines the bias at which device 6! starts to be operative, and by suitable choice of tube 6! and connections, to determine the effectiveness of this device in altering the load impedance on the rectifier, any desired relation between curves H and 72 may be realized. In particular, with rectifier M of relatively high impedance, and potentiometer 53 set to be operative after a small'change of bias. and with potentiometer 35 set somewhat higher than for curve 12, a substantially uniform expansion curve, as shown in H may be realized. Or as another illustration, rectifier 61 may be chosen or connected for very low impedance when passing current,

contact 53 may be set for limiting action when the bias on tubes l21.is slightly more than for curve H, and potentiometer 35 set high to cause rapid change of bias with respect to the input signal. In this case, the curve 14 shows the resulting relation between input and output decibels. This curve depicts conditions suitable for speech reproduction in which the output dynamic range is not required to'be so great as for orchestral music, and the device is especially adapted to procure silence and freedom from ground noise in the absence of speech.

Other combinations of control may be used for different purposes, and for certain applications;

controls may be fixed for purposes of simplicity. While certain novel features of the invention have been disclosed and are, pointed out in the annexed claims, it will be understood that various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A system for reproducing sound from a sound record including a pair of stages of amplification arranged in cascade, a common control circuit for each of said stages and means for operating said control circuit in accordance with the-average value of the input signal for varying the amplification factors of said stagesas an increasing function of said average value, said control circuit causing a greater variation in the first of said stages than in the following stage, and means for limiting the change in gain ratio when loud signals are applied so that overloading is prevented.

2. A system for reproducing sound from a sound record comprising a plurality of stages of amplification connected in cascade, the first of said stages being operated at a lower amplificationfactor than the second of said stages, a control circuit for varying the amplification factors of said stages in proportion to the average value ,of the applied signal in such manner that the volume range of the signal is expanded, the amplification factor of said first stage being varied over wider limits than that of said second stage, and means for limiting the change in gain ratio when loud signals are applied so that overloading is prevented.

3. A system for reproducing sound from a sound record including a pair of stages of amplification arranged in cascade, means for normally applying a bias to. each of said stages for controlling the amplification factors thereof, the bias on the first stage being greater than that on the subsequent stage, a control circuit for reducing said bias in proportion to the average value of. the input signal, the change of bias in said first stage being greater than the change in bias in said subsequent stage, and means for limiting the change in gain ratio when loud signals are applied so that overloading is prevented.

4. A system for reproducing sound from a sound record comprising a plurality of stages of amplification, each stage including a space discharge amplifier having a control element, impedances in series with said control elements, a control circuit associated with said impedances for passing a current therethrough proportional to the strength of the input signal and in a direction such that the potential drops in the various impedances oppose the normal biases of said control elements, the relationship being such that an increased signal strength decreases the normal negative biases of said control elements and causes ranged in cascade, a series of resistances common in part to all of said amplifiers and in series with their control elements, a control circuit associated with said resistances for passing a current therethrough which is dependent upon the inputsignal strength of the first amplifier, whereby the volume range of each stage of said amplifier is expanded, and means for preventing the voltage drop in said resistances from exceeding a predetermined value.

6. A system for reproducing sound from a sound record comprising a plurality of stages of amplification, each stage including a space discharge amplifier having a control element, impedances in series with said control elements, a control circuit associated with said impedances for passing a current through said impedances proportional to the strength of the input signal and in a direction such that the potential drops in the various impedances oppose the normal biases of said control elements, the relationship being such that an increased signal strength decreases the normal negative biases of said control elements and causes an increase in the amplification ratio of the space discharge devices, said increase in amplification being different for each of said devices, and means for preventing said negative biases from decreasing below a predetermined value when strong signals are applied.

7. In a system for use with a sound record, a space discharge amplifier, a control circuit, means for impressing the signal onto said amplifier and control circuit, an impedance in said control circuit for determining the gain ratio of said amplifier, means for causing a current to flow through said impedance which varies in accordance with the average value of said impressed signal, and a shunt circuit around said impedance adapted to pass current when the voltage thereacross exceeds a predetermined amount.

8. In a system for use with a sound record, a

space discharge amplifier, a control circuit, means for impressing the signal onto said amplifier and control circuit, an impedance in said control circuit for determining the gain ratio of said amplifier, means for causing a current to flow through said impedance which varies in accordance with the average value of said impressed signal, anda shunt circuit comprising a rectifier and a. voltage source around said impedance, said shunt circuit being adapted to pass current when the voltage thereacross exceeds the voltage of said source.

9. In a sound record system. a pick-up device, an amplifier fed thereby, a control circuit controlled by the energy received from said pick-up device for varying the gain ratio of said amplifier, said control circuit comprising an impedance in the input circuit of said amplifier, a capacitance and a rectifier, and a vacuum tube discharge device for receiving discharge from said capacitance.

10. In a sound record system, the method of amplifying sound which comprises expanding the volume ratio of the sound, fixing the initial gain with no signal, fixing the ultimate gain with maximum signal and variably controlling the change in gain between said initial gain and ultimate gain to obtain the desired expansion characteristics.

11. In a sound record system, the method of amplifying sound which comprises expanding the volume ratio of the sound and controlling with change in signal strength the change in gain between initial and ultimate gain according to the desired pattern.

12. In a sound system using a sound record, a pick-up device, an amplifier fedby said pickup device, a translating device fed by said amplifier, a control channel for controlling the gain of said amplifier according to signal strength, means for presetting the initial bias on said amplifier with no signal, means for presetting the ultimate bias for maximum signal, devices for variably governing the law of change in bias between said initial and ultimate bias with change in signal strength, and means for presetting said devices.

JOHN HAYS HAMMOND, JR. 

